Offshore structures are in constant need of protection from the corrosive environment of sea water. The useful life of offshore steel structures such as oil well drilling and production platforms and piping systems can be severely limited by the corrosive environment of the sea. Conventional protection against such damage adds considerable complication and weight to offshore structures.
Cathodic protection by either sacrificial anodes or impressed current is generally effective in preventing corrosion on fully submerged portions of an offshore structure. In some offshore locations, such as the North Sea, oxygen content is relatively high even in water depths to 1,000 feet. As a consequence, oxidative corrosion is very severe and can readily occur at these depths.
Installation and maintenance of sacrificial anodes adds greatly to the weight and expense of an offshore structure. This is particularly true with respect to a tension leg platform. In a tension leg platform, high-strength, thick walled steel tubulars are constantly maintained in tension between their anchor points on the ocean floor in a floating structure whose buoyancy is constantly in excess of its operating weight. The use of high-strength steel in a tension leg platform for fabricating the mooring and riser elements is necessitated by the desire to reduce the platform displacement and minimize the need for complicated heavyweight tensioning and handling systems. The mooring and riser systems are subjected to more than 100,000,000 floating cycles during a common service life for a tension leg platform. This makes corrosion and, particularly, corrosion fatigue resistance an important design parameter. Therefore, the selection of a corrosion protection system that achieves long term corrosion protection and minimizes the influence of the sea water environment on fatigue resistance is essential to insure the intergrity of the high-strength steel components.
The most common approach to corrosion protection involves the use of aluminum anodes. Such a system has the disadvantage that the cathodic potential on the steel with respect to such aluminum anodes approaches minus 1,050 mV versus a saturated calomel electrode (SCE). This cathodic level can result in hydrogen embrittlement in the high-strength steel used in the structural components,. Testing has shown that a cathodic potential below negative 800 mV (SCE) subjects the high-strength steel to hydrogen embrittlement thereby limiting the crack resistance and fatigue life of the structural elements.
Additionally, a reliable electrical contact must be maintained between a sacrificial anode and the high-strength steel. The electrical attachment method must not impair the mechanical or metallurgical performance of the steel. Mechanical electrical connections are generally not reliable and not recommended for long term use. Brazing and thermite welding can enhance the potential for stress corrosion crackng of highstrength steel. Friction welding of an aluminum stud to a high-strength steel has also been shown to cause failure in test specimens with cracks initiated either under the stud or at the edge of the weld.
An impressed current system often involves throwing current from anodes in relatively remote locations with respect to the structure to be protected. The distance between anodes and remote components can be too great for effective control of the impressed current, particularly at remote locations such as the anchor end of a tension leg mooring system.
For protection of high-strength steel components such as the mooring and riser systems for TLP's, the use of inert coatings cannot be seriously considered without the addition of cathodic protection because of the inevitable damage to and water permeation of the coatings through the life of the platform. Also, some areas of the components have tolerances that do not permit coating. With coatings, the size of the required sacrificial anodes would be greatly reduced by the electrical connection and hydrogen embrittlement problems would be present.
A coating of flame-sprayed aluminum has been proposed for use in marine environments. Such a coating offers the advantage of relatively high bond strength and a uniform potential of about minus 875 mV (SCE). Such flame sprayed aluminum coatings overcome the problems of electrical connection as well as hydrogen embrittlement which are present with aluminum anode cathodic protection systems.
While flame sprayed aluminum coatings appear to solve all of the potential problems with respect to cathodic protection of marine structures, the common method of applying such flame sprayed aluminum coatings can lead to problems affecting the life of the protected structure. Specifically, a flame sprayed aluminum coating generaly requires a roughened "anchor" on the steel substrate to which it is to be applied.
The anchor pattern may be provided by scoring the steel surface or, most commonly, provided by sand or grit blasting to provide a roughened surface. The surface discontinuities induced by these anchor patterning provisions introduce sites which offer increased potential for fatigue cracking during the life of the structural component. The overall fatigue strength of the component can thus be reduced.
The porous nature of a flame sprayed aluminum coating offers additional potential for marine biofouling and, therefore, must be sealed in order to avoid problems associated with biofouling.